Acoustic sound relates to frequencies of sound that are perceptible to the human ear (20 Hz and 20 kHz). Acoustics is the scientific study of the behaviour of mechanical sound waves in various forms of matter, e.g. gases, liquids and solids. Like any mechanical or energy wave, moving acoustic wave fronts can be refracted or reflected as they encounter a new medium. In the same way light is refracted on moving from an air or liquid medium to a glass medium, sound waves are refracted on moving from an air or liquid medium to a glass one. Interestingly and importantly for the present invention, sound waves, unlike light (which moves linearly), propagate spherically; thus the orientation of a detecting means to the sound source in the system under study is relatively inconsequential, as the acoustic data contained in energy that reaches the detecting means contains all the data of the total system. Typically a microphone is used to detect and record the sound profiles.
Sonochemistry, the chemistry of sound, generally refers to the use of sound, principally ultrasound (sound above 20 kHz), to trigger chemical reactions which are difficult to achieve under normal atmospheric conditions. In simple terms BARDS technology is a new direct method of monitoring the liberation and subsequent release of dissolved gases from solution during the dissolution of a solute. It must be stressed that the platform presented herein has little, if any, connection to sonochemistry, where ultrasound is actively used to drive reactions or monitor their progression.
Minor investigations were carried out on the acoustic effect of dissolving a solute into a liquid (W. Bragg et al) in the 1930s. Further investigations were made by F. Crawford and details were published in his papers “The Hot Chocolate Effect (1981)” and “Hot water, Fresh Beer and Salt (1985)”. They described the acoustic effect as resulting from a decrease in the resonant tone of the glass, followed by a prolonged increase in the tone. The effect was linked to the formation of microscopic bubbles in solution. These bubbles are thought to generate as a result of the addition of a solute, which through dissolution, forces a proportional quantity of gas out of solution.
The so called “Hot Chocolate Effect” is a phenomenon of wave mechanics, where the pitch heard when a cup of hot liquid is tapped, rises after the addition of a soluble powder.1 The “Hot Chocolate Effect” works for any liquid in which a gas is soluble. When the vessel containing the liquid is tapped to resonate and a solute is introduced, it is observed that the velocity of sound in the liquid is reduced below that of bubble-free liquid and the pitch of the emitted sound is correspondingly reduced. As the bubbles liberated by the dissolving solute float to the top of the vessel, a smaller fraction of the volume has reduced sound velocity and so the pitch rises, until eventually the pitch corresponds to that of bubble-free liquid wherein the majority of the bubbles have floated to the top.
Crawford postulated that the bubbles, at first homogeneously distributed throughout the solution, begin to rise and create a layer of bubble-filled liquid, which gradually reduces in size as the bubbles exit the liquid phase through the surface. The gradual increase in pitch results from the reduction of the bubble layer, or rather the increasing volume of liquid unaffected by desolvating gas. This allows sound to travel faster through the clear layer, due to lower compressibility, thus producing a higher pitched note. Eventually all produced gas, due to desolvation, is eliminated from solution and the resonant frequencies of the vessel return to steady state.
As a solute dissolves in a solution, the solubility of dissolved gases reduces. The velocity of sound is reduced as the gas bubbles exit the solution with a corresponding reduction in frequency (pitch). Crawford noted that two opposing parameters control this phenomenon: mass density “e” (inertial property) and compressibility “c” (elastic property). Sound travels more slowly the greater the mass density of the gas or liquid, e.g. sound travels faster in helium than it does in air. However, greater compressibility of a medium also results in the reduced velocity of sound.
Even though the inertial factor favours gases, the elastic factor has a greater influence on the speed (v) of a wave. Therefore, the velocity of sound in a solid>velocity in liquid>velocity in gases according to the following equation:v=1/√{square root over (e·c)}  Equation 1
where v=velocity, e=mass density, and c=compressibility of the liquid. Aqueous solutions have a mass density 800 times that of air that imparts a reduced sound velocity compared to air based on inertial properties. In comparison, air is 15,000 times more compressible than water, thus air carries sound more slowly by a factor of 4.3. In a solution containing gas bubbles, the two factors combine to significantly reduce the speed of sound. This is due to the greater mass density of the solution, which also has temporarily the compressibility of a gas.
The effect is also observed in hot liquids, e.g., when air under high pressure in a solution of hot water comes out of solution, it forms bubbles. The greater the air in the form of bubbles in the solution, the lower the pitch of sound emitted when a vessel holding the water is tapped continuously to allow the vessel to resonate. No effect is observed when cold water is used. This is because the dissolved air stays in solution.
To date, despite the phenomena being known for some time, no analytical techniques have been developed to make use of the effect.
Therefore, it is desirable to provide new, versatile methods of materials analysis capable of employing broadband resonance spectroscopy and an analytical instrument to carry out such analysis.